On Mon, 21 Apr 2008 20:22:17 +0300, Iain Churches wrote:
<snip>
>
> Can I suggest a topic for you? A current limiter for tube heaters.
> People are keen to use voltage regulation, which doesn't seem to me to
> be so important. Heaters are specified at +/- 5% so anything between 6V
> and 6V6 for a 6V3 heater is within tolerance, and with a wirewound pot
> between two 'lytics one can get it spot on. But heaters draw huge
> currents at switch on. Do you have a circuit, using those funny black
> things with shiny legs, that could current limit? Two circuits would be
> of interest. One at say 1.2A (a pair of B9A's) and one at 3A (a pair
> of EL34's)
>
<snip>
Here's hoping the ASCIIart works...
+
----------,
| |
R1 heater
| |
| c
+------b TR2
| e
c |
TR1 b------+
e |
| R2
| |
----------'
-
+/- is from a 6.3v winding, bridge rectified & with a reservoir cap. It
should be about 7.8v in theory. TR1 is a small transistor such as BC107.
TR2 needs to carry the load so will need to be suitably rated. Both are
NPN.
R1 is only used to turn TR2 on. Value depends on TR2 hFE, but probably
about 150-220R.
R2 is calculated based on the current required. 0.6/I so for 1.2A use
0.5R. For 0.3A use 2R. For 3.0A use 0.2R. These won't be exact, but near
enough for jazz!
Note that because of the conversion from AC to DC you will need to have
more current capability in the transformer than you need for AC heaters.
The advantage over a lot of the heater circuits that I've seen is that it
only has about 0.8v drop, so the low supply voltage from a 6.3v winding
should be acceptable.
Now the disclaimer: I haven't actually tested this... :-)
--
Mick (Working in a M$-free zone!)
Web: http://www.nascom.info
http://mixpix.batcave.net
Filtering everything posted from googlegroups to kill spam.
|
